Many nucleic acid sequences have clinical relevance. For example, nucleic acid sequences associated with infectious organisms provide indications of the presence of an infection by the organism. Nucleic acid sequences not normally expressed in a patient sample may indicate activation of pathways associated with a disease or other conditions. Still other nucleic acid sequences may indicate differences in a patient's likely response to proposed therapies.
Determination of clinically relevant nucleic acids generally depends on controlled amplification of specific nucleic acid sequences and detection of the amplification products. Amplification improves analytical sensitivity by generating sufficient copies of nucleic acids found in the sample for ready determination. Amplification may also improve analytical specificity by selectively generating only those nucleic acids of clinical interest. A problem with amplification-based determinations, particularly when amplification generates large numbers of copies of a target nucleic acid sequence, is the possibility that some of these copies from one sample might contaminate other samples to produce apparently elevated results where none of the target nucleic acid sequence was originally present in the sample.
Other sources of contamination could affect nucleic acid determinations. Carryover between samples can contribute contaminating material. An amplification mixture may receive contaminating materials from environmental sources transferred on surfaces or by laboratory technicians or by aerosols. In some cases, unintended transfers of reagents, such as inappropriate amplification primers, may contaminate mixtures and cause erroneous results. Amplification mixtures may also retain interfering substances originally present in the sample through incomplete purification of target nucleic acids. Thus, there is a need for automation of nucleic acid analysis that avoids transfer and retention of contaminating material from a variety of sources.
Clinical laboratory workflow is a consequence of medical care delivery and varies between institutions. A clinic or large group practice may generate patient specimens throughout the course of a day at a relatively constant rate. In contrast, a clinical reference laboratory may receive all of its specimens in one or two deliveries and a large hospital may generate specimens through a large blood draw in the morning supplemented by an irregular stream of samples throughout the day. Most nucleic acid analysis specimens arrive at a clinical laboratory in a sequence unrelated to the type of requested assay. In some cases, selected specimens may be of high priority with immediate or critical treatment decisions dependent on the outcome. Other specimens may be of more routine priority. Non-specimen samples such as laboratory controls may be interspersed among the clinical specimens according to individual laboratory practice. In some cases, exhaustion of reagents or of particular lots of reagents may dictate the insertion of controls and calibration samples irrespective of other samples in queue.
Thus, there is a need for an analytical system having flexible and adjustable operating capabilities to match the unpredictable demand of clinical laboratories.
Nucleic acid analysis determines multiple analytes from diverse source organisms using a mix of specimen types. These inputs drive diverse processing requirements. For example, RNA and DNA have different chemical properties and stabilities; their preparation may use different processing regimens, different enzymes, and different thermal conditions. Both the base sequence and the length of target analytes affect binding energy, and hence processing. The length and sequence of complementary oligonucleotides used for amplification further affect amplification conditions.
Different source organisms for analytical targets may require different steps to release or isolate the nucleic acid sequences. For example, release of DNA sequences from gram positive bacteria might use elevated temperatures not used for release of DNA sequences from relatively labile white blood cells.
Thus, there is a need for an analytical system able to freely intermix a variety of processing protocols, each composed of a variety of processing steps. Technologies exist that attempt to address some of the issues described above.
Russel/Higuchi in U.S. Pat. No. 5,994,056, Homogeneous Methods for Nucleic Acid Amplification and Detection, described improved methods for nucleic acid detection using methods such as the polymerase chain reaction (PCR). Higuchi described methods for simultaneous amplification and detection to enhance the speed and accuracy of prior methods. The methods provide means for monitoring the increase in product DNA during an amplification reaction. According to the description, amplified nucleic acids are detected without opening the reaction vessel once the amplification reaction is initiated and without any additional handling or manipulative steps subsequent to the reaction.
K. Rudi et al. described a Rapid, Universal Method to Isolate PCR-Ready DNA Using Magnetic Beads in BioTechniques 22(3) 506-511, March 1997. Rudi et al. described application of a magnetic bead-based kit for rapid DNA isolation (Dynabeads® DNA DIRECT™; Dynal A. S.) to diverse organisms and tissues to produce a general approach for the purification of PCR-ready DNA. DNA suitable for PCR was prepared in less than 30 minutes.
Systems that automate nucleic acid analysis have a long history. Integrated platforms demonstrated the entire range of automated analytical and preparative steps, including isolation of nucleic acid, amplification of the isolated material, and detection of the amplification products.
For example, Bienhaus et al. in U.S. Pat. No. 5,746,978, Device for Treating Nucleic Acids from a Sample, described a single device to link treatment steps that separate nucleic acids from other sample components with steps for amplification of the nucleic acids. The device included reaction chambers for individual treatment steps with the outlet of one chamber attached to inlet of another. A conventional pipetting instrument transferred both the nucleic acid-containing sample liquid and all possibly necessary reagents from sample and reagent storage containers into the device. Bienhaus et al. described magnetic separation, amplification by PCR or NASBA, and using a hybridization probe complementary to the PCR amplificate in a detection reaction measured using an ES analyzer (manufactured by Boehringer Mannheim).
P. Belgrader, et al. described Automated DNA Purification and Amplification from Blood-Stained Cards Using a Robotic Workstation in BioTechniques 19(3) 427-432 1995. Belgrader et al. introduced a prototype which could perform coupled DNA purification and amplification that required no user participation once the process was initiated. The method was implemented into a high throughput automated system using a Biomek® 1000 robotic workstation (Beckman Instruments) using phenol and isopropanol to purify DNA on blood-stained cards. The Biomek® 1000 performed DNA purification and amplification using an HCU (Biomek® on-board heater-cooler unit) as a thermal cycler. Belgrader et al. described that the next objective was to integrate a detection step for a completely automated DNA typing system.
Patrick Merel et al. described Completely Automated Extraction of DNA from Whole Blood in Clinical Chemistry 42, No. 8, p 1285-6 1996. Merel et al. disclosed using the Biomek® 2000 (Beckman Instruments) and DNA DIRECT™ (Dynal France S. A.) in combination to fully automate the DNA extraction procedure using magnetic particle separation. Merel et al. used several different PCR protocols to evaluate the quantity and quality of the DNA obtained. Merel et al. routinely used the described materials for a 10-min automated DNA extraction procedure, a 10-min automated PCR setup step for 96 tubes, PCR for 80 min, and a simple electrophoresis analysis of 15 min.
Ammann et al. U.S. Pat. No. 6,335,166 Automated Process for Isolating and Amplifying a Target Nucleic Acid Sequence described an automated analyzer including multiple stations, or modules, in which discrete aspects of the assay are performed on fluid samples contained in reaction receptacles. The analyzer includes stations for automatically preparing a specimen sample, incubating the sample at prescribed temperatures for prescribed periods, preforming an analyte isolation procedure, and ascertaining the presence of a target analyte. An automated receptacle transporting system moves the reaction receptacles from one station to the next. Ammann also describes a method for performing an automated diagnostic assay includes an automated process for isolating and amplifying a target analyte. The process is performed by automatically moving each of a plurality of reaction receptacles containing a solid support material and a fluid sample between stations for incubating the contents of the reaction receptacle and for separating the target analyte bound to the solid support from the fluid sample. An amplification reagent is added to the separated analyte after the analyte separation step and before a final incubation step.
Even though such automated systems have been available, further improvements are desirable. In particular, multiple sources of contamination continues to risk erroneous results. Further, complexities of multi-step processes needed for complete nucleic acid analysis can produce processing bottlenecks and degrade repeatability, limiting answer reporting turnaround and processing flexibility. Limited answer reporting turnaround may increase the time to institute proper clinical treatment. Lack of processing flexibility limits support for variations in assay protocols for a broad and expandable test menu. Lack of processing flexibility may also force laboratories to sequence or batch samples and reagents in a manner at odds with clinical need.
Embodiments of the invention address these and other problems, individually and collectively.